1. Field of the Invention
The present invention generally relates to systems using transponders, that is, transceivers (generally mobile) capable of communicating in a contactless and wireless manner with a terminal. The present invention more specifically relates to systems using active transponders provided with a rechargeable battery.
2. Discussion of the Related Art
Two categories of transponders are generally distinguished according to whether or not they are equipped with a battery.
So-called passive transponders extract the power necessary to the operation of the electronic circuits that they comprise from a high-frequency field radiated by a terminal with which they communicate. Such transponders are generally reserved for transmissions within a short range (on the order of a few meters). The storage of the power received from the terminal is performed by a capacitor connected to the transponder antenna.
So-called active transponders to which the present invention more specifically applies comprise a battery providing the power necessary to their operation. Such transponders are capable of operating at a longer range (up to a few hundreds of meters) since they need no power transfer between the terminal and the transponder.
Active transponders which take advantage of situations in which they are at a short distance from a terminal to recharge their battery are also known.
For example, document EP-A-0999517 (or U.S. Pat. No. 6,462,647) describes a rechargeable active transponder equipped with a storage capacitor for storing power originating from a received radio-electric signal, charge means being provided to recharge a battery from the power stored in the capacitor.
Document U.S. Pat. No. 6,944,424 describes an electronic tag capable of being powered by an internal battery and, passively, by a radio-frequency field. A power storage device is coupled to the battery to enable its recharge.
The power originating from the electromagnetic coupling between the transponder and the terminal which is capable of being used to recharge the transponder battery is often insufficient and/or requires too long a recharge time to be effective. As a result, in many cases, an active transponder stops in practice to operate at long distance once its original battery has been discharged, unless this transponder is left for a long time within the range of a terminal to be recharged, which is not always easy.
For example, in an automobile vehicle where the transponder is the vehicle key, the key remains at a short distance from the terminal contained by the vehicle all along travels. It can thus be recharged during this period. However, when the vehicle is not in use, the key is no longer within the range of the vehicle terminal and is thus not recharged.
In other applications, it is almost impossible for the transponder to remain for a long time within the range of the terminal with which it is supposed to communicate. For example, for chip cards used in transports, the time period for which the transponder is within the range of a terminal generally corresponds to the time to perform an information exchange and does not always allow to properly recharge a battery. Further, for reasons of anti-collision of the exchanges between the many transponders and terminals which are, in this type of application, close to one another, the communication (and thus the recharge) is only set up in very close coupling (less than 10 centimeters), which in practice amounts to having to lay the card on the terminal's housing in a designated area corresponding to the position closest to the antenna.
Further, transponders of a given type are generally only capable of being recharged from dedicated terminals, which is not always convenient.
EP-A-1 154 367 discloses a terminal for communicating with an electromagnetic transponder and capable of validating the presence of a transponder in its field. This document provides modifying the value of a capacitive element of the oscillating circuit to modify its tuning.